Polishing slurry and method for chemical-mechanical polishing

ABSTRACT

A method for polishing a grid of a field emission display (FED) with a polishing slurry contain very small particle of colloidal particles of amorphous silica in an alkaline medium. The method results in highly selective planarization well-suited for chemical-mechanical polishing (CMP) of the grid for the self-aligned CMP-FED fabrication process. An FED grid made according to this method is also disclosed.

This application claims benefit to Provisional Application No.60/051,520 filed Jul. 2, 1997.

TECHNICAL FIELD

The present invention relates to a polishing slurry and method for usingthe same in chemical-mechanical polishing, particularly in thefabrication of field emission displays.

BACKGROUND OF THE INVENTION

Chemical-mechanical polishing (“CMP”) processes are widely used toremove material from the surface of a substrate in the production of awide variety of microelectronics. In a typical CMP process, the surfaceto be polished is pressed against a polishing pad in the presence of aslurry under controlled chemical, pressure, velocity, and temperatureconditions. The slurry generally contains small, abrasive particles thatabrade the surface, and chemicals that etch and/or oxidize the newlyformed surface. The polishing pad is generally a planar pad made from acontinuous phase matrix material such as polyurethane. Thus, when thepad and/or substrate moves with respect to the other, material isremoved from the substrate surface mechanically by the abrasiveparticles and chemically by the etchants and/or oxidants in the slurry.

FIG. 1 schematically illustrates a polishing machine 10, often called aplanarizer, used in a conventional CMP process. The polishing machine 10has a platen 20, substrate carrier 30, a polishing pad 40, and a slurry44 on the polishing pad. An under-pad 25 is typically attached to theupper surface 22 of platen 20, and the polishing pad 40 is positioned onthe under-pad 25. In conventional CMP machines, a drive assembly 26rotates the platen 20 as indicated by arrow A. In other existing CMPmachines, the drive assembly 26 reciprocates the platen 20 back andforth as indicated by arrow B. The motion of the platen 20 is impartedto the pad 40 through the under-pad 25 because the polishing pad 40frictionally engages the under-pad 25. The substrate carrier 30 has alower surface 32 to which a substrate 12 may be attached, or thesubstrate 12 may be attached to a resilient pad 34 positioned betweenthe substrate 12 and the lower surface 32. The substrate carrier 30 maybe a weighted, free-floating carrier, or an actuator assembly 36 may beattached to carrier 30 to impart axial and rotational motion, asindicated by arrows C and D, respectively.

In the operation of the conventional polishing machine 10, the substrate12 is positioned face-downward against the polishing pad 40, and thenthe platen 20 and the carrier 30 move relative to one another. As thesurface of the substrate 12 moves across the planarizing surface 42 ofthe polishing pad 40, the polishing pad 40 and the slurry 44 polish thesurface of the substrate.

CMP processes must consistently and accurately produce a uniform, planarsurface. The necessity of obtaining a highly uniform, planar surface isillustrated in the manufacture of microelectronic devices manufacturedon a substrate made from glass or a suitable semiconductive material(e.g., silicon) on a suitable insulating material (e.g., glass). Suchmicroelectronic devices typically have many small components formed inmultiple layers of materials. One type of microelectronic deviceparticularly relevant to the present invention is a field emissiondisplay (“FED”).

FEDs are one type of flat panel display in use or proposed for use incomputers, television sets, camcorder viewfinders, and a variety ofother applications. FEDs have a baseplate with a generally planaremitter substrate juxtaposed to a faceplate. FIG. 2 illustrates aportion of a conventional FED baseplate 20 with a conductive emittersubstrate 30, and a number of emitters 32 formed on the emittersubstrate 30. An insulator layer 40 made from a dielectric material isdisposed on the emitter substrate 30, and an extraction grid 50 madefrom polysilicon is disposed on the on the insulator layer 40. A numberof cavities 42 extend through the insulator layer 40, and a number ofholes 52 extend through the extraction grid 50. The cavities 42 and theholes 52 are aligned with the emitters 32 to open the emitters 32 to thefaceplate (not shown).

Referring to FIGS. 2 and 3, the emitters 32 are grouped into discreteemitter sets 33 in which the bases of the emitters 32 in each set arecommonly connected. As shown in FIG. 3, for example, the emitter sets 33are configured into rows (e.g., R₁-R₃) in which the individual emittersets 33 in each row are commonly connected. Additionally, each emitterset 33 has a grid structure superjacent to the emitters that isconfigured into columns (e.g, C₁-C₂) in which the individual gridstructures are commonly connected in each column. Such an arrangementallows an X-Y addressable array of grid-controlled emitter sets. The twoterminals, comprising the emitters and the grids, of the three terminalcold cathode emitter structure (where the third terminal is understoodto be the anode disposed on the faceplate—not shown in FIG. 2 or 3) arecommonly connected along such rows and columns, respectively, by meansof high-speed interconnects. The interconnects 60 (also shown in FIG. 2)are formed on top of the emitter substrate 30 and the extraction grid50, and they serve to electrically connect the individual gridstructures forming the columns. It will be appreciated that the columnand row assignments were chosen for illustrative purposes and can beexchanged.

In operation, a specific emitter set is selectively activated byproducing a voltage differential between the extraction grid and thespecific emitter set. A voltage differential may be selectivelyestablished between the extraction grid and a specific emitter setthrough corresponding drive circuitry that generates row and columnsignals that intersect at the location of the specific emitter set.Referring to FIG. 3, for example, a row signal along row R₂ of theextraction grid 50 and a column signal along a column C₁ of emitter sets33 activates the emitter set at the intersection of row R₂ and columnC₁. The voltage differential between the extraction grid and theselectively activated emitter sets produces localized electric fieldsthat extract electrons from the emitters in the activated emitter sets.

The display screen of the faceplate (not shown) is coated with asubstantially transparent conductive material to form an anode, and theanode is coated with a cathodoluminescent layer. The anode, which istypically biased to approximately 1.0-2.0 kV, draws the extractedelectrons through the extraction grid and across a vacuum gap (notshown) between the extraction grid and the cathodoluminescent layer ofmaterial. As the electrons strike the cathodoluminescent layer, lightemits from the impact site and travels through the anode and the glasspanel of the display screen. The emitted light from each of the areasbecomes all or part of a picture element.

FIGS. 4A-4C illustrate a prior art method for forming an FED baseplate120. FIG. 4A illustrates an initial step in which a plurality ofemitters 32 are formed on an emitter substrate 30. The emitters 32 arepreferably grouped into discrete emitter sets, and the emitter sets arepreferably configured into columns or rows on the emitter substrate 30,as discussed above with respect to FIG. 3. The emitters 32 arepreferably conical-shaped protuberances that project upwardly from theemitter substrate 30 towards a faceplate (not shown). The shape of theemitters 32, however, is not limited to conical protuberances and may beany other suitable shape. The emitter substrate 30 is typically madefrom conductive silicon. Alternatively, the emitters 32 may be formedfrom a conductive layer (not shown) that was deposited on an insulatingsubstrate such as glass.

FIG. 4B illustrates a subsequent stage in the method for forming the FEDbaseplate 120. After the emitters 32 are formed on the emitter substrate30, an insulator layer 40 is deposited over the emitter substrate 30 sothat the insulator layer 40 generally conforms to the contour of theemitters 32 and the false-emitter defect 34. A unitary interconnect/gridlayer 70, which is preferably made from a material having a conductivitysufficient to operate the FED at a refresh rate of 60 Hz, is thendeposited over the insulator layer 40. Suitable materials from which theinterconnect/grid layer 70 may be made include, but are not limited to,aluminum, copper, or tungsten. The interconnect/grid layer 70 ispreferably deposited to a thickness of 0.5 to 5.0 μm. Since, theinterconnect/grid layer 70 is deposited over the insulator layer 40before the insulator layer 40 is planarized with a CMP process, theinterconnect/grid layer 70 generally conforms to the contour of theinsulator layer 40.

The actual conductivity of the interconnect/grid layer 70 depends uponseveral factors, some of which are: (1) the current draw of theemitters; (2) the inductance and capacitance of the extraction grid; (3)the shape and size of the extraction grid; (4) the number of grey scalesof the display; and (5) the color spectrum of the display. In a specificexample, which is not intended to limit the scope of the invention, theconductivity value of the interconnect/grid layer 70 is preferably lessthan or equal to 500 (ohm-cm)⁻¹ for a display with the followingparameters: (1) an active display area of 12.1 inches as measured acrossthe diagonal; (2) a VGA resolution (640×480 lines); (3) full-on spatialcolor RGB display format supporting 256 grey scales; (4) a refresh rateof 60 Hz; and (5) a passive drive scheme with horizontal rows addressingthe interconnect/grid layer and vertical columns addressing theemitters. In general, since the intersection of an addressed row andcolumn activates the emitters at that particular pixel and the length oftime that the emitters are biased controls the grey scale of theparticular pixel, the interconnect/grid layer 70 is made from a materialhaving a minimum conductivity sufficient to transmit signals tosubstantially all commonly connected grid segments with a refreshinterval of at least approximately 10-40 μsec.

FIG. 4C illustrates the baseplate 120 after the interconnect/grid layer70 and insulator layer 40 have been planarized with a CMP process. ToCMP the interconnect/grid layer 70 and the insulator layer 40, the frontside of the baseplate 120 is pressed against a chemical-mechanicalplanarization polishing pad (as discussed above) in the presence of aslurry under controlled chemical, pressure, velocity and temperatureconditions. The slurry generally contains small, abrasive particles thatabrade the front face of the baseplate, and chemicals that etch and/oroxidize the materials on the front face of the baseplate. The polishingpad is generally a planar pad made from a continuous phase matrixmaterial, and abrasive particles may be suspended in the matrixmaterial. Thus, when the pad and/or the baseplate move with respect tothe other, material is removed from the front surface of the baseplatemechanically by the abrasive particles and chemically by the etchantsand/or oxidants.

The CMP process is endpointed (see FIG. 4C) so that a number of holes oropenings 72 are formed in the interconnect/grid layer 70 over theemitters 32 without exposing the tips 36 of the emitters 32. This methodof FED manufacture is known as self-aligned CMP-FED fabrication process.FIG. 5 illustrates a completed baseplate 120 with a number of cavities42 formed in the insulator layer 40 adjacent to the emitters 32. Thecavities 42 are preferably formed by a subsequent wet etch process thatis selective to the material of the insulator layer 40.

Referring again to FIG. 4B, the CMP process should have a high disparitybetween polish rates on structured surfaces versus smooth surface. Inother words, during the CMP process, it is desirable to have the highestpeaks, designated 80 in FIG. 4B, polished at a higher rate than thevalleys between peaks, designated 82. In this manner, planarizationefficiency is very high, permitting large surface areas to beeffectively polished to endpoint by CMP processes.

Accordingly, there is a need in the art for an improved polishing methodwhich permits highly efficient CMP planarization, particularly in thecontext of FED manufacturing, and which provides well-controlled polishrate across large surface areas. The present invention fulfills thisneed, and provides further related advantages.

SUMMARY OF THE INVENTION

The invention provides a method for polishing the grid of an FEDutilizing an aqueous polishing slurry containing very small colloidalparticles of amorphous silica in an alkaline medium. The polishingslurry of this invention yields highly selective planarization—that is,the small colloidal particles in an alkaline medium provide awell-controlled polish rate across large areas, and with a very highdisparity between the polish rates on structured versus smooth surfaces,allowing large surfaces to be polished to endpoint effectively.

In the practice of this invention, the polishing slurry is preferablyused to polish the grid of in the context of a self-aligned, CMP-FEDfabrication process. The polishing slurry contains 20% to 50% by weightcolloidal particles of amorphous silica having an average diameterranging from about 12 μm to about 50 μm, and a sufficient amount of analkaline compound to such that the pH of said slurry is above about 9.5.In one embodiment, the polishing slurry further contains an ammoniastabilizer.

In a further aspect, a CMP-FED grid is disclosed made according to themethod of this invention.

These and other aspects of the will be evidence upon reference to theattached figures and following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a chemical-mechanicalplanarization machine in accordance with the prior art.

FIG. 2 is a partial schematic isometric view of an FED baseplate inaccordance with the prior art.

FIG. 3 is a partial plan view of the FED baseplate of FIG. 2 inaccordance with the prior art.

FIG. 4A is a partial schematic cross-sectional view of an FED baseplateat one point in a method for making the baseplate; FIG. 4B is a partialschematic cross-sectional view of the FED baseplate of FIG. 4A atanother point in a method for making the baseplate; and FIG. 4C is apartial schematic cross-sectional view of the FED baseplate of FIG. 4Bat another point in a method for making the baseplate.

FIG. 5 is an partial isometric view of the of the FED baseplate of FIG.4C.

DETAILED DESCRIPTION OF THE INVENTION

As mentioned above, this invention provides a method for using apolishing slurry in chemical-mechanical polishing (CMP), particularly inthe fabrication of a field emission display (FED). It has beensurprisingly discovered that a polishing slurry containing smallcolloidal particles in an alkaline medium provide a well-controlledpolish rate across large surface areas. The disparity between polishrates on structured versus smooth surfaces (i.e., the planarizationefficiency) is very high, allowing large surface areas to effectively bepolished to endpoint.

In the practice of this invention, and as discussed in greater detailbelow, a preferred polishing slurry is sold commercially under theproduct designation 30N25 by Solutions Technology, Inc. (Monroe, N.C.).Such polishing solutions typically contain small colloidal particles ofamorphous silica having an average diameter of about 25 μm in analkaline medium having a pH of about 10.

More generally, polishing solutions of this invention are aqueoussolutions contain a suitable amount of a base such that the pH of thesolution is above about 9.5, and preferably from about 10.5 to about11.5. Suitable bases include, but are not limited to, potassiumhydroxide and sodium hydroxide. The polishing solutions also containsmall colloidal particles of amorphous silica having a diameter rangingfrom about 12 nm to about 50 nm, and preferably about 25 nm. Thecolloidal particles are present in the polishing slurry in an amountranging from 20% to 50% by weight, and typically about 30% by weight

The polishing slurry of the invention may optionally contain furthercomponents, such as a stabilizer. In one embodiment, the optionalcomponent is an ammonium ion stabilizer or tetramethylammonium hydroxide(TMAH).

A polishing slurry of this invention may be prepared by combining (e.g.,mixing) the components set forth above, or by utilizing commerciallyavailable slurries. Representative polishing slurries of this inventionmay be purchased commercially from Solutions Technology, Inc. (Monroe,N.C.) under the product name KEBOSOL® and product numbers “30 N 50 pHN”,“30 N 25” and “PL 1506”. Such slurries are opalescent liquidscharacterized as colloidal suspensions of amorphous silica in alkalinemedium, with a solids concentration of about 30% by weight and anaverage particle size of 50 nm for 30 N 50 pHN and PL 1506, and 25 nmfor 30 N 25. The stabilizer in all three products is ammonia, with a NH₃content of 0.1% to 0.3% by weight, and the manufacturing base is sodiumsilicate for 30 N 50 pHN and 30 N 25, and potassium silicate for PL1506. The pH of these slurries is 11, 9.5-10.5 and 10.5-11.0 for 30 N 50pHN, 30 N 25 and PL 1506, respectively.

While not intending to be bound by the following theory, it is believedthat the finer sized particles which are in suspension (and thus notsusceptible to clumping) are more effective at polishing the CMP-FEDtopography. To this end, it has been found that polishing the 1.0-1.5 μmhigh points or “bumps” (see peaks 80 of FIG. 4B) is best accomplishedwith particles sized 40-100 times smaller compared to existing largeparticle slurries, where the particle diameter of existing slurriesapproaches the size of the CMP-FED bump.

Accordingly, in the practice of this invention, the above slurry isemployed in the polishing of an FED surface in a CMP process. To thisend, the surface to be polished is pressed against the polishing surfaceof polishing pad 40, as shown on the CMP machine 10 in FIG. 1. Thedown-force between the surface and the polishing surface of thepolishing pad is typically between 5 and 50 psi. The polishing slurry isdeposited on the polishing pad, and the surface and polishing pad aremoved with respect to each other to impart relative motion therebetween.As the surface and the polishing pad move with respect to one another,the polishing pad and polishing slurry results in a high planarizationefficiency—that is, the polishing rate between the high points on thesurface (see peaks 80 of FIG. 4B) are polished at a much higher ratethan the low points on the surface (see valleys 82 of FIG. 4B), whichpermits relatively large surfaces of the FED to be effectively polishedto end point.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

What is claimed is:
 1. A method for chemical-mechanical polishing of afield emission display grid, comprising polishing said grid with anaqueous polishing slurry containing 20% to 50% by weight colloidalparticles of amorphous silica having an average diameter ranging fromabout 12 nm to about 50 nm, and a sufficient amount of an alkalinecompound to such that the pH of said slurry is above about 9.5.
 2. Themethod of claim 1 wherein the colloidal particles have an averagediameter of about 25 nm.
 3. The method of claim 1 where the aqueouspolishing slurry comprises about 30% by weight colloidal particles. 4.The method of claim 1 wherein the alkaline compound is selected frompotassium hydroxide and sodium hydroxide.
 5. The method of claim 1wherein the alkaline compound is sodium hydroxide.
 6. The method ofclaim 1 wherein the pH of the slurry ranges from 9.5 to 12.0.
 7. Themethod of claim 1 wherein the pH of the slurry ranges from 10.5 to 11.0.8. The method of claim 1 wherein the polishing slurry further comprisesammonium ion stabilizer.
 9. The method of claim 8 wherein the ammoniumion is present in the slurry in an amount ranging from about 0.1% to0.3% by weight.
 10. A method for chemical-mechanical polishing of afield emission display grid, comprising polishing said grid with anaqueous polishing slurry containing about 30% by weight colloidalparticles of amorphous silica having an average diameter of about 25 nm,an ammonium ion stabilizer, and a sufficient amount of an alkalinecompound such that the pH of said slurry ranges from 9.5-11.0.
 11. Apolished field emission display grid made according to the method of anyone of claims 1-10.